1. Field of the Invention
The present invention relates to a projection type display apparatus utilizing a spatial light modulator (light valve), and more particularly, to a projection type display apparatus having the structure for securing optically stable performance against influence of external stress and effectively restricting image quality degradation of projection image.
2. Related Background Art
A known example of the spatial light modulator (light valve) conventionally used for projection type display apparatus is a spatial light modulator of a phase difference modulation type (polarization modulation type), which spatially modulating light, utilizing polarization. For example, a modulator formed using the liquid crystal (phase difference modulation type liquid crystal light valve) is practically available as the spatial light modulator of the phase difference modulation type.
The conventional projection type display apparatus using such a spatial light modulator of the phase difference modulation type uses a polarizing beam splitter (primary polarizing beam splitter) serving as a polarizer and an analyzer. For the below description, let us assume that the polarizing beam splitter has such a property as to reflect an s-polarized light component and transmit a p-polarized light component. In the conventional projection type display apparatus, the polarizing beam splitter splits light incident thereinto (light directly incident from an illumination light source or light after color-separated before incidence thereto) into light of the p-polarized light component and light of s-polarized light component, and normally, the s-polarized light component out of the light thus split into is projected to the spatial light modulator. After modulated and reflected by a liquid crystal layer of the spatial light modulator, the light again goes back into the polarizing beam splitter. On this occasion, the reflected light from the spatial light modulator 4 is analyzed by the polarizing beam splitter. The above polarizing beam splitter reflects the s-polarized light component, but transmits only the p-polarized light resulting from the modulation by the spatial light modulator. The transmitted light (that is, the analyzed light) is projected as a projection image through a projection optical system onto a screen or the like.
Many polarizing beam splitters and color separating/combining optical systems used in such projection type display apparatus have been proposed as liquid immersion type optical components in such structure that a plate of an optically transparent material coated with a coating for polarizing beam splitter and a plate of an optically transparent material coated with a color separating dichroic film were respectively immersed in a liquid with adjusted refractive index, as disclosed for example in U.S. Pat. No. 4,687,301 owned by Hughes Aircraft Co. The refractive index of the above liquid, at a predetermined temperature of the liquid, is adjusted so as to be equal to that of the plate of the transparent material. The reason why the beam splitter is immersed in the liquid is that, supposing the beam splitter were set in air, the interface of the coating would be in a relation of air against transparent material and the polarizing beam splitter would fail to function because of a difference in refractive index.
In the conventional projection type display apparatus, the light incident into the polarizing beam splitter is split by the polarizing beam splitter into the p-polarized light component and s-polarized light component, among which the s-polarized light component is projected to the color separating/combining optical system. After the color separating/combining optical system separates the incident s-polarized light component into some color components, the separated color components are respectively modulated by liquid crystal layers and respectively reflected by reflection layers of light valves which are prepared in accordance with the color components. The modified color components are combined by the color separating/combining optical system, and thereafter the combined light returns to and is analyzed by the polarizing beam splitter. The analyzed light is projected as a projection image through the projection optical system onto the screen or the like.